1. Field of the Invention
This invention relates to power transmission systems and, more particularly, to a system which employs a V-ribbed belt to drive and/or be driven by a plurality, and preferably at least three, pulleys. The inventive system exhibits excellent drive characteristics with minimal noise generation.
2. Background Art
Power transmission systems utilizing V-ribbed belts are desirable because of their ability to positively and efficiently transmit a large force in a compact space. Because of these characteristics, the V-ribbed power transmission system has a wide range of applications. One exemplary application for such systems is in the automotive industry. A single belt has been used to replace a plurality of belts which are used to simultaneously drive various engine accessories off the engine crankshaft.
In the prior art multi-belt power transmission system, a plurality of belts are trained, one each around a pulley on the engine crankshaft and a shaft carried by each accessory to be driven during engine operation. To accommodate the numerous accessories, this type of system requires that the engine crankshaft be extended sufficiently in an axial direction to accommodate the same number of pulleys as there are accessories to be driven. As a result, the engine crankshaft becomes quite lengthy. In turn, the engine compartment must be made sufficiently large to accommodate this extended crankshaft.
The recent trend has been to move away from the above conventional system to the single belt system. In this system, commonly referred to as a serpentine drive system, a single belt, as disclosed for example in U.S. Pat. No. 4,028,955 and Japanese Patent Laid-Open No. 50-49563, is employed. The belt wraps around all of the accessories that are to be driven by the crankshaft. The single belt system simplifies the engine construction by requiring only a single pulley on the crankshaft. The crankshaft extension required to accommodate the pulley is minimal. The engine may resultingly be made relatively compact, and the compartment size required to accommodate the engine can be reduced over the earlier described conventional construction.
In the most basic serpentine belt system, a V-ribbed belt is wrapped around various pulleys which are in a fixed position. The components are adjusted at assembly to place the desired tension on the belt. However, because the belt is very long and relatively thin, it has a tendency to elongate during use. The result of this is that the tension applied to the belt is reduced, which may result in slippage and, in a worse case, disengagement of the belt from the pulleys.
To avoid this belt slackening, it is known to use an auto-tensioner with an idler pulley thereon that is biased, as by a spring, against an unsupported portion of the belt between adjacent pulleys with a predetermined tension. The auto-tensioner compensates for elongation of the belt.
There is an additional problem that is associated with the V-ribbed belt, that is not contended with with conventional V-belts. The long V-ribbed belt, operating on an automobile engine, is subjected to a large load variation as the engine starts, stops and is accelerated. This load variation tends to induce vibrations to the system which may result in rough operation and/or unwanted noise generation.
To diminish the effects of vibration, auto-tensioners have been constructed with assemblies to damp these vibrations, in addition to applying a predetermined tension to the belt. Various damping structures have been employed. In one exemplary system, a sliding member is translated against typically a disk-shaped friction generating surface by a spring element, that is separate from a torsion spring of the type normally used to pivot the support arm carrying the idler pulley in a direction to tension a belt. This generates a constant friction force on the belt during use. This type of system is disclosed in Japanese Patent Application No. 63-258870. The teaching therein is that the auto-tensioner should have a damping ratio of not less than 30%, with the damping ratio being expressed by the following formula: 2.times.[(A-B)/(A+B)].times.100, where A and B respectively indicate forces exerted during up-stroking and down-stroking of the idler pulley.
Heretofore, the serpentine systems utilizing a V-ribbed belt, in combination with an auto-tensioner, have been able to stably transmit power without the belt disengaging from the pulleys. However, with these systems, there has been an undesirably high level of noise generation during acceleration and deceleration, and particularly at startup of the engine.
It is a common understanding among those skilled in the power transmission system art that noise from transmission belts occurs particularly when the tension of the belt has decreased. In those power transmission systems which do not employ an auto-tensioner, it has been the practice to increase the tension on the V-ribbed belt to suppress the generation of noise. However, it has been demonstrated that a power transmission system employing an auto-tensioner generates an undesirably large amount of noise when the engine speed is varied, as during startup of the engine, even when the belt tension is kept constant through the auto-tensioner.